How does striated muscle contract?

Striated muscle, also known as skeletal muscle, contracts through a complex process involving the interaction of actin and myosin filaments, the basic building blocks of muscle fibers. The contraction process is triggered by nerve impulses that cause the release of calcium ions from the sarcoplasmic reticulum, the muscle cell's internal calcium storage. Here is a step-by-step explanation of how striated muscle contracts:

1. Nerve Impulse:

- A nerve impulse reaches the muscle fiber, causing the release of acetylcholine, a neurotransmitter, into the synaptic cleft (the gap between the nerve and muscle cells).

- Acetylcholine binds to specific receptors on the muscle cell membrane, leading to the generation of an action potential.

2. Action Potential:

- The action potential travels along the muscle cell membrane, causing the invagination of the membrane at specific sites called transverse tubules (T-tubules).

3. Calcium Release:

- The T-tubules are closely associated with the sarcoplasmic reticulum, which stores calcium ions.

- The action potential causes a conformational change in the T-tubules, leading to the opening of calcium channels on the sarcoplasmic reticulum.

- Calcium ions flood out of the sarcoplasmic reticulum and into the sarcomere (the contractile unit of the muscle fiber).

4. Calcium Binding to Troponin:

- Inside the sarcomere, calcium ions bind to a protein called troponin, which is part of the troponin-tropomyosin complex.

- This binding causes a conformational change in the troponin-tropomyosin complex, exposing a binding site on the actin filament.

5. Myosin Head Binding to Actin:

- Myosin, a motor protein, has two globular heads that can bind to and hydrolyze ATP (adenosine triphosphate), the energy currency of the cell.

- With calcium bound to troponin, the myosin heads can now bind to the exposed binding sites on the actin filament.

6. Power Stroke:

- Once bound to actin, the myosin heads undergo a conformational change, causing them to pivot and pull the actin filament toward the center of the sarcomere.

- This movement is known as the power stroke and results in the shortening of the sarcomere.

7. Cross-Bridge Formation and Sliding Filament Mechanism:

- The power stroke leads to the formation of cross-bridges between the myosin heads and the actin filaments.

- As long as calcium is present and ATP is available, the myosin heads will continue to bind to actin, undergo power strokes, and release actin, causing the filaments to slide past each other.

- This sliding filament mechanism shortens the sarcomere and generates muscle contraction.

8. Relaxation:

- When the nerve impulse ceases and calcium levels decrease in the sarcopere, calcium ions are actively pumped back into the sarcoplasmic reticulum.

- Without calcium bound to troponin, the troponin-tropomyosin complex returns to its original conformation, blocking the myosin binding sites on actin.

- Myosin heads detach from actin, and the sarcomere relaxes.

By repeating this process, striated muscles can contract and relax, allowing for controlled movement and various physical activities.